1. Field of the Invention
The invention relates generally to devices for sorting ripe from unripe comestibles, and for sorting comestibles from extraneous, inorganic matter. More specifically, the color sorter herein uses a pulsating, multiple frequency illumination source in conjunction with a synchronous detection system to collect data about the reflectivity responses of the subject objects. The data is subsequently analyzed by comparator and logic circuitry to determine whether a particular object is acceptable or unacceptable. Acceptable objects are passed on for further processing, whereas unacceptable objects are actively removed from the object stream and discarded.
2. Prior Art
It has long been recognized that materials exposed to light wave energy at a selected frequency reflect the incoming light at a characteristic exposure/reflectance amplitude ratio, depending upon the nature or condition of the material. This principle can be used to advantage for sorting purposes where a particular comestible exhibits a light reflectivity response in a ripe and mature condition which differs substantially from the reflectivity response of the same comestible in an unripe and immature condition. Likewise, where an organic comestible does not reflect light in the same fashion as extraneous, inorganic material, such as dirt, a comparison based upon the light reflectance ratios can determine the nature of the material examined.
While a single test frequency can be used, a dual frequency, or biochromatic system provides a far improved signal to noise ratio between the data signals used to make object determinations. The two test frequencies are selected to exhibit a respective amplitude peak or dip in the reflectivity response of the object, depending upon whether the object displays a desirable or an undesirable characteristic.
For example, an immature tomato exhibits a reflectivity peak at approximately 550 nm and a relatively low reflectivity response at 660 nm. On the other hand, a ripe tomato displays a large upswing in reflective respose around 660 nm, and a sharp dip in reflective response at 550 nm. By comparing the reflectivity characteristics of a particular tomato at these two selected frequencies, the comparator circuitry of a tomato sorter will be provided with data having a substantially better signal to noise ratio than if the reflective response were tested at solely one frequency.
It is also evident that more than two test frequencies can be used to good advantage, where the additional frequencies correspond to other characteristics, desirable or undesirable. U.S. Pat. No. 4,120,402, issued to Swanson, shows the use of four test frequencies in a color sorter. Two of the frequencies therein were selected to make a ripe/unripe determination, and two additional frequencies were devoted to detecting and sorting out inorganic, extraneous material.
It is further significant to note that the sorter in Swanson uses a constant amplitude object illuminator, having a broad spectral output. The reflected light can properly be considered an object modulated version of the incident light. The reflected light is then sensed concurrently by four detectors, each fitted with a filter designed primarily to pass light wave energy at one of the four selected frequencies. Consequently, the data output of each detector represents a direct analog of the article's passage through the field of view. In short, the illumination/detection system in Swanson makes object determinations as a substantially constant, or DC frequency.
Sunlight also displays a characteristic broad spectral output, and therefore extraneous sunlight in the field of view leads to unreliable operation of a DC based color sorter. Multiple detectors within a single channel react differently to varying temperatures encountered during in field operations. This is especially true where near and deep infrared test frequencies are used in conjunction with temperature sensitive lead sulfide detector cells. In summary, there are a number of shortcomings inherent in multiple frequency DC based color sorters.
A quite different approach is taken in a color sorter using an AC based, or pulsed light wave illumination/detection system. In an AC based system, the object is sequentially exposed to light wave pulses at a number of discrete frequencies, and a single unfiltered detector senses the reflected light. The output of the detector does not represent a direct analog of the object's presence in the field of view, as in the DC based detector system. Rather, the output of an AC based detector system produces a series of pulses, corresponding to a considerable number of illumination sampling cycles. Averaged over a period of time, the detector output pulses relating to a particular frequency approximate the reflective response of the article under test.
The use of an AC based illumination/detector system permits the use of a single detector per channel, which for reasons to be discussed herein, decreases both the light and temperature susceptibility of the color sorter. Furthermore, the use of pulsed light wave energy also allows the data to be processed and analyzed with much more efficiency and flexibility than with DC based systems. U.S. Pat. No. 4,369,886, jointly invented by Lane, the applicant herein, is representative of an AC based color sorter. The present invention represents an improvement over the known prior art, including that cited above, in its illumination/detector assembly and in its synchronous comparator/logic circuitry.